Recently, ceramic lasers that use polycrystalline materials as laser media have been attracting attention. For example, the laser medium in the ceramic lasers is produced by sintering a raw-material powder in a vacuum after press forming the raw-material powder into a predetermined configuration (See Patent Literature No. 1, and Non-patent Literature No. 1, for instance).
For the laser medium in these ceramic lasers, materials that are isotropic optically have been used. And, as for an optically isotropic material, YAG polycrystalline bodies that have a cubic-system crystalline structure have been used mainly. These cubic-system YAG polycrystalline bodies, one of the optically isotropic materials, exhibit an identical refractive index with respect to all directions. Consequently, they function effectively as the laser medium in the same manner as YAG single-crystalline bodies do.
Moreover, cubic-system polycrystalline bodies, such as Y2O3 and Sc2O3, have been used for the laser medium, in addition to the YAG.
In this way, as for the laser medium in the conventional ceramic lasers, the cubic-system polycrystalline bodies have been studied and developed solely to arrive at being put into practical use. This is because of the following: Since the polycrystalline bodies are constituted of a large number of microscopic single-crystalline particles, it is only possible to make polycrystalline bodies alone that scatter lights so greatly that they are unsuitable for the laser medium even when ordinarily forming and then sintering optically anisotropic single-crystalline particles that have a crystal-orientation dependency in the refractive index.
Meanwhile, apatite-system crystals (e.g., FAP, SFAP, SVAP, etc.), vanadate-system crystals (e.g., YVO4, etc.), and the like, are useful as gain media, because they can enhance the oscillation efficiency in solid-state laser. However, the apatite-system crystals belong to the hexagonal system, and the vanadate-system crystals moreover belong to the tetragonal system. That is, the apatite-system crystals, vanadate-system crystals (e.g., YVO4, etc.), and so forth, are materials that are anisotropic optically. Consequently, although these optically anisotropic materials are useful extremely as gain media, single-crystalline materials have come to be selected inevitably in a case of being used as the laser medium.
However, although the single-crystalline materials are produced by means of single crystal growth such as the Czochralski process, it has problems in the processing because it takes time for the growth in this single crystal growth, in addition to having strict limitations on the size and configuration of makeable samples.
In contrast to this, when it becomes feasible to make anisotropic media by means of sintering, one of the methods for making polycrystalline material, it is possible to greatly contribute to furthermore expanding laser technologies, because it becomes easier relatively to make large-size vanadate media or large-size apatite media that are unmakeable with the single crystals, although they have been needed in order for the realization of high power laser.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 5-235,462; and    Non-patent Literature No. 1: Annu. Rev. Mater. Res. 2006.36: pp. 397-429, “Progress in Ceramic Laser,” Akio Ikesue, Yan Lin Aung, Takunori Taira, Tomosumi Kamimura, Kunio Yoshida, and Gary L. Messing